In the drilling of wells into the earth by rotary drilling techniques, a drill bit is attached to a drill string, lowered into a well, and rotated in contact with the earth; thereby breaking and fracturing the earth and forming a wellbore thereinto. A drilling fluid is circulated down the drill string and through ports provided in the drill bit to the bottom of the wellbore and thence upward through the annular space formed between the drill string and the wall of the wellbore. The drilling fluid serves many purposes including cooling the bit, supplying hydrostatic pressure upon the formations penetrated by the wellbore to prevent fluids existing under pressure therein from flowing into the wellbore, and hole cleaning (carrying capacity), i.e. the removal of drill solids (cuttings) beneath the bit, and the transport of this material to the surface through the wellbore annulus.
As summarized in COMPOSITION AND PROPERTIES OF OIL WELL DRILLING FLUIDS, 4th Edition, G. R. Gray, et al., Gulf Publishing Company, 1980, the term "drilling fluid" includes systems in which the principal constituent can be a gas, water, or oil, and the formulation injected through the drill bit may be as simple as a dry gas, "fresh water", lease crude, or as complex as a slurry, colloidal dispersion, emulsion, foam or mist containing oil and/or water, viscosifier, fluid loss additive, electrolytes, polymers, weighting material, surfactant, corrosion inhibitor, oxygen scavanger, defoamer, etc. Those drilling fluids generally inapplicable to this invention include the dry gas, mist, and "fresh water".
Factors directly and indirectly affecting the efficiency of removal of drill solids (cuttings) from the wellbore include (1) drilling fluid rheology, density, and chemical composition, (2) drilling conditions, e.g. drilling fluid circulation rate, bit rate of penetration, drill string rotational speed, available hydraulic horsepower, (3) drill solids characteristics, e.g. density, mineralogy, size, shape, strength, and (4) wellbore and drill string configuration and characteristics, e.g. inclination of the wellbore, dimensions of the annular channel and the drill string, eccentricity between the drill string and the wellbore and borehole stability.
A measure of the efficiency of the hole cleaning operation is the difference between the annular fluid velocity (V.sub.A) and the terminal (slip) velocity (V.sub.S) at which the largest cutting settles divided by the annular fluid velocity. The equation for determining transport ratio (TR) is ##EQU1## where V.sub.A =annular fluid velocity
V.sub.S =terminal (slip) velocity.
Obviously total removal of drill solids would correspond to a transport ratio of 100 percent, however, this degree of efficiency can be difficult to achieve because of practical constraints on the factors enumerated above. Thus in practice it is customary to set some minimum value to this transport ratio based on experience in drilling operations in a certain area, or to relate the ratio to the maximum concentration of drill solids to be permitted in the annulus between the drill string and the wellbore wall.
The present invention provides a method for increasing the transport ratio, thereby improving the hole cleaning operation, by manipulating the rheological characteristics of the applicable drilling fluids described above. The particular rheological parameters manipulated are the plastic viscosity and yield point of liquid materials exhibiting plastic flow, parameters described by Savins in U.S. Pat. No. 2,703,006, the disclosure of which is hereby incorporated by reference, together with mathematical procedures for extracting same from viscometric data on said drilling fluid systems. More specifically, the present invention provides a method for hole cleaning while drilling a well wherein the transport ratio is increased by controlling the yield point and plastic viscosity of the drilling fluid in a prescribed manner.